Method of and apparatus for the combustion of sludge

ABSTRACT

A method of and an apparatus for the combustion of sludge obtained from a clarifier in the purification of water, wherein the clarifier sludge is passed through a multistage drier above a fluidized-bed furnace in which it is predried with the waste mass of the latter. As the wastes dry, they are comminuted and distributed over the cross section of the fluidized-bed furnace for burning therein.

United States Patent 1 1 Menigat Nov. 20, 1973 [54] METHOD OF ANDAPPARATUS FOR THE 2,286,309 6; 1942 Rowen 110/12 L E 2,138,120 11 19 30Raisch 110 12 COMBUSTION OF S UDG 2,213,668 9/1940 Dundas et a1 l10/15Inventor: Richard g Dletzenbach, 3,351,030 ll/l967 Albertson et a1. 1107 Germany 3,515,381 6/1970 Foch 110/8 X [73] Assignee:Mettalgesellschatt AG, Frankfurt am Germany Primary Examiner-Kenneth W.Sprague [22] Filed: July 9, 1971 AttarneyKarl F. Ross [21] Appl. No.:161,071

57 ABSTRACT [30] Foreign Application Priority Data 1 July 29, Germany P20 37 561.5 A method of and an apparatus for the combustion of sludgeobtained from a clarifier in the purification of U.S. e0 R, waterwherein the clarifier sludgq is a 51 11 1. c1. F23g 7/00 tistag fliabove a fluidized-bed furnace in Fled of Search R, 8 R, 12, is predriedthe waste mass of the latter 0/15 As the wastes dry, they are comminutedand distributed over the cross section of the fluidized-bed fur-References Cited nace for burning therein.

UNITED STATES PATENTS Smith et a1 0/8 7 Claims, 2 Drawing FiguresPATENTEDauv 20 ms 3.772.998 SHEET 1 CF 2 SZ4VVAA1 19 17. Up-"1.7, 6c

Inventor: Richard Menigaf {km Attorney Pmgmguuuvzo ma 3372.998 SHEH 2 PF2 Richard Menigaf Inventor.

Attorney METHOD OF AND APPARATUS FOR THE COMBUSTION OF SLUDGE FIELD OFTHE INVENTION My present invention relates to a method of and anapparatus for the burning of clarifier sludge and, more particularly, toa system for disposing of the sludges recovered as wastes inwater-purification plants.

BACKGROUND OF THE INVENTION In the treatment of sewage or organicallycontaminated water, especially domestic, municipal or industrial sewage,it is a known practice to introduce flocculants and other materials intothe raw water or a sewage which may or may not be oxygenated, to causeagglomeration of the organic matter concurrently with or subsequently tosome biological degradation or breakdown thereof. In a subsequent stage,the the solids are settled from the water which is decanted, can befiltered and generally is treated with a disinfectant. The water may besubjected to several such settling and solid-removal stages, dependingupon the degree of contamination. Since the solid wastes containmicroorganisms which are effective in breaking down the wastes containedin contaminated water, it is common practice to contact the latter witha sludge derived from previous processing operations. Suchactivated-sludge techniques promote removal of organic contaminants fromthe sewage or raw waste of the type previously described.

After contact with the sludge, the water-sludge slurry may be passedinto a clarifier in which the sludge is settled for partial or totalrecirculation, while the purified water is decanted. Sludge may also beseparated by filtering and other techniques.

Disposal of the sludge has been a problem in many processes and it is,for example, possible to treat the sludge so that it contains nopathogenic or harmful microorganisms and can be effectively used as afertilizer. Frequently, however, the additional treatment of the sludgerequired to be converted into a useful product is expensive to the pointthat the economy of the entire water-treatment or sewage-treatmentsystem may be in jeopardy. In such cases a less expensive disposal ofthe sludge is desired. It has been proposed, in this connection, to burnclarifier sludges. The difficulties involved in the combustion ofclarifier sludges are considerable, owing to the physical property ofthe sludge as it is removed from the clarifier. Firstly, clarifiersludges are often obtained in a consistency or viscosity ranging from athin flowable liquid to a thick viscous liquid, to a nonflowable paste.Furthermore, the consistency or viscosity of the sludge may vary withinrelatively short periods. To facilitate combustion of the sludge,therefore, it has been proposed to eliminate excess water by adewatering process in a filter or centrifuge, by a drying process inwhich water is evaporated, or by a combination of both. The systems areextremely expensive both to operate and to construct, since dewateringarrangements invariably require arrangements for treating the sludge,recovering and storing the filter cake, handling the filter cake etc.The process is timeconsuming and requires considerable labor and spaceand has the disadvantage that the sludge or waste must come in contactwith operating personnel and thereby create a health hazard.

Many prior-art proposals have similar or additional disadvantages. Forexample, there have been suggestions which may not yet have become stateof the art, for the combustion of sludge in multiple-hearth furnaceswherein, toward the discharge end of the furnace, combustion is effectedwhile drying takes place at the charging end. In such systems, the wastematter or sludge moves counter to the combustion and exhaust gases, thelatter serving to dry and preheat the sludge before it moves into thecombustion stage. In such systems, excessively moist sludge, i.e.,sludge with a high water content, may be detrimental to the processbecause it may be insufficiently dried and preheated before entering thecombustion stage and may interfere with the combustion process. On theother hand, if the moisture content of the sludge is low, the heat ofthe exhaust gases may be used inefficiently or insufiiciently so thathot exhaust gases are discharged from the system and the heat losses areconsiderable. High exhaustgas temperatures may damage the flue, thedustseparating systems downstream of the flue, or the chimney.

In order to eliminate some of these disadvantages, systems fordewatering the sludge to a constant moisture content have been proposed.These systems have the disadvantage that an additional apparatus isrequired and that dewatered sludge must be transferred from thedewatering apparatus to the furnace. The system, generally usingconveyors, creates unsanitary or noisome conditions. Furthermore, it isdifficult to control the feed rate or furnace when a dewateringarrangement is provided in the line because of the inherent time lags.

Still another suggestion which is intended to avoid the disadvantages ofmultiple-hearth furnaces charged with wet sludge, makes use of a totaldrying of the sludge before it is introduced into the multiple-hearthfurnace. This system has the obvious disadvantage that the drying stageis an expensive addition to the plant and has thermal requirements whichmay be only partly supplied by the combustion process. Exhaust gas andclarifier sludge energies are utilized with low efficiencles.

I may also mention briefly two other combustion processes which have notbeen found to be fully practical for the combustion of sludge. The firstis a fluidizedbed system in which wastes may be burned but which must becharged with specially prepared materials, must be provided with specialnozzles and which is difficult to control. The other is a raked-bedprocess, whereby materials drying in an upper portion of the furnace iscascaded downwardly into a furnace chamber below. While fluidized-bedfurnaces have the advantage that the burned product does not evolvesmoke or smell which is released to the atmosphere, and the advantagethat control is more convenient, all prior-art efforts to introduce wet,moist or pasty sludge have proved to be unsatisfactory unless dewateringsteps have been practiced and the wastes transformed into a highlyviscous state. The term highly viscous is used herein to revert to asubstantially thixotropic condition in which the sludge is a paste whichretains its shape. For example, a hole pierced in alayer of sludge willnot be self-closing. In all of the processes of the latter type,difficulties have been encountered with the distribution of the wastematerial over the cross-section of the combustion chamber and suchnonuniforrnity is characterized, on the one hand,'by sintering or bakingofthe material where overheating occurs, and of unsatisfactorymaintenance of the fluidized bed. The fluidized bed breaks down or isrendered inhomogeneous as a result of such overheating and irregularcombustion may occur which may result in termination of the combustionprocess.

OBJECTS OF THE INVENTION It is the principal object of the presentinvention to provide an improved method of burning clarifier sludge andlike materials of variable viscosity whereby the aforementioneddisadvantages can be obviated.

It is another object of the invention to provide an apparatus for thecombustion of sludge which is readily controlled, is free from theinhomogeneity characterizing earlier systems, is highly efficient andcan operate with maximum heat utilization.

Still another object of this invention is the provision of a method ofand an apparatus for the combustion of sludge which enables the startingmaterial to be relatively viscous or of low viscosity, as required,without detrimentally affecting the burning process.

SUMMARY OF THE INVENTION I have found, surprisingly, that theabove-mentioned disadvantages can be avoided, eliminated or amelioratedin a sludge-burning method and apparatus in which a fluidized bed formsthe combustion zone and is surmounted by a multistage drying andpreheating zone such that the exhaust gases of the combustion zone risedirectly through the multistage zone above the fluidized bed. Animportant feature of this invention is the fact that the multistagedrying and preheating zone is constituted in the form of amultiple-hearth furnace with annular supporting surfaces, eachassociated with one or more rotary brakes or agitators whch continuouslybreak up the layer on the respective stage and maintain a movement ofthe sludge downwardly from one stage to the next until a dried,preheated, comminuted, solid material is distributed over the entirecross-section of the fluidized bed.

Hence it is essential to the present invention that the fluidizing gasescontain oxygen so as to effect a combustion in the bed, that the wastesare comminuted and transformed into a particulate or other subdividedform. during the drying and preheating process, and that the comminuteddry matter is distributed over the grate area of the fluidized-bedfurnace.

As already noted, the simultaneous drying and comminuting of the wastesis effected in a multiple-hearth furnace having a plurality ofvertically spaced, coaxial annular hearths heated with the risingexhaust gases from the fluidized-bed furnace which is disposed directlytherebelow. This multiple-hearth furnace can be much shorter than thosewhich have been proposed hitherto for the combustion of wastes and mayhave fewer hearths than the multiple-bed combustion furnaces knownearlier. In fact, three to five hearths have been found to be sufficientfor most purposes.

As the exhaust gases from the fluidized-bed furnace pass through themultiple-hearth furnace, gases may be generated by thermal evolutionfrom the wastes as a result of distillation or decomposition; thesegases may have a detrimental effect or may produce an undesirable odorin the region of the plant. Accordingly, I provide feed-back means forrecycling exhaust gases from the multiple-hearth furnace into thefluidized-bed furnace, so that the only gases discharged into theatmosphere are the exhaust gases of thefluidized-bed furnace. The ratioof the exhaust gas portion supplied to the drying and preheating hearthsto the exhaust gas discharged through the flue may vary within widelimits, and preferably between 30 and 60 percent by volume of theexhaust gas produced in the fluidized bed is used in the drying zone.The high temperatures of the fluidized-bed furnace, of course, preventnoisome and detrimental combustionable constituents from remaining inthe exhaust gases above the fluidized bed.

According to the apparatus aspects of the present invention, themultiple-hearth furnace is of the type in which the annular, verticallyspaced, axially superposed hearths are cantilevered from the inner wallof the housing of the drying zone and define a central space in which ashaft is rotatable, this shaft carrying a plurality of radial arms ateach stage, the arms having depending plate-like rake teeth for scrapingthe deposit on the annular platform overhung thereby. The teeth may beprovided at various inclinations to the rotation circles defined by thearms, i.e., lie along tangents or chords to induce a movement of thecomminuted waste spirally inwardly or outwardly depending upon thelocation of the outlet for each platform. The platforms are alternatelyprovided with outlets along the inner periphery or the outer peripheryso that, for example, the comminuted drying material may be induced toflow inwardly at one annular platform, is caused to cascade downwardlyonto an intercepting inner peripheral portion of the next-lowerplatform, is induced to move outwardly along this latter platform by theassociated rake or comminution arms, and is caused to cascade downwardlyalong the outer periphery of this platform onto still another platformunderlying same.

In addition, the apparatus comprises a fluidized-bed furnace which formsthe combustion zone and is surrounded by the multiple-hearth furnace,while a transfer conduit is provided for recycling the waste gases ofthe multiple-hearth furnace into the fluidized-bed furnace.Surprisingly, the action of the rake teeth produces a comminutedmaterial which is highly combustible and easily strewn across the grateof the fluidized bed, or, more specifically, across the fullcross-section of the fluidized-bed furnace with uniform distribution.

DESCRIPTION OF THE DRAWING The above and other objects, features andadvantages of the present invention will become more readily apparentfrom the following description, reference being made to the accompanyingdrawing in which:

FIG. 1 is a vertical (axial) cross-sectional view through a sludgecombustion plant according to the present invention; and

FIG. 2 is a cross-section taken along the line II II of FIG. 1.

SPECIFIC DESCRIPTION In the drawing, I have shown a plant P for thecombustion of sludge which comprises a generally cylindrical furnaceshaft 1 having an outer metal shell and an inner refractory lining. Theshaft 1 has a substantially cylindrical portion la defining a chamber 8above a fluidized bed 5 maintained in a downwardly convergentfrustoconical portion lb of the furnace shaft 1 directly below thecylindrical portion la. A grate 2 spans the base of the fluidized bed 5and may be of any conventional construction, e.g. slotted grate, capgrate, nozzle grate, etc. Such grates are known in the fluidized-bed artand can be provided directly above a wind box which forms a manifold ordistributing chamber (which is temperature controlled TR) through whichan oxygen-containing gas is introduced into the furnace.

Fluidizing air is supplied to the bed by a blower 6 through a valve 6aand enters at the burner 3 of a combustion chamber 4 adapted to maintainthe temperature of the fluidized bed at the desired level. A fuel, e.g.,oil or gas, is introduced into the burner 3 by a line 1 l and iscontrolled by a valve 11a to heat the gases traversing the chamber 4 toa temperature of 500 to 600C. The fuel proportion is stoichiometricallyinsufficient to consume all of the oxygen in the gas traversing thechamber 4 and I prefer to maintain the proportion such that oxygen isstoichiometrically in excess over the quantity required for totalcombustion in the fluidized bed and for reaction with the fuel. Tomaintain heat balance, one or more lances 7 may open into the fluidizedbed 5 for directly supplying oil or gas thereto as controlled by a valve7a and a temperature regulator 7b responsive to the temperature at theflue 9 of the apparatus.

Surmounting the fluidized bed is a multiple hearth drying and preheatingfurnace 13 having the individual hearths 13a, 13b, 13c and 13d best seenin FIG. 1. The hearths 13a to 13d are cantilevered from the inner wallof the furnace shaft 1 and are composed of a refractory or ceramicmaterial and have cross-sections which taper inwardly. The hearths areall annular to define a central opening in which a hollow shaft 15 isreceived. Furthermore, the upper part 13a has a central opening 19through which the sludge is permitted to pass downwardly while thenext-lower hearth 13b has peripheral openings 20 forming downcomers forthe sludge. A central opening at 19a of the next-lower hearth 13c andopenings 21 of the lowest part 13d complete the path for the solidwastes.

The predried and comminuted wastes enter the fluidized bed incountercurrent to the exhaust gases and cascade downwardly with ascattering angle represented at 12 to be distributed uniformly over theentire cross-section of the fluidized bed.

The fluidized bed, composed of quartz sand, is maintained at atemperature of 900C or thereabove so that the solid wastes burnspontaneously upon reaching the bed. The exhaust gases rise incountercurrent to the waste. A portion of the gases pass through ports21 as shown by arrows D to enter the drying furnace while the remainderis discharged through the flue 9 which is located in the fluidized bedportion of the furnace.

The waste is charged onto the first hearth of the furnace by a meteringdevice 14a, here shown to be a screw conveyor, receiving the comminutedmaterial from a hopper, as represented at 14. Since the initialdowncomer 19 is located at the inner periphery of the hearth 130, themetering device 14a opens downwardly at 14b onto the outer periphery ofthis hearth. In place of a screw conveyor, I may use a valve, belt orbucket conveyor or the like, with a gas-sealing arrangement forpreventing the venting of undesirable gases to the atmosphere.

The hollow shaft 15 carries at each level a plurality of generallyradial rake arms16 whose stirring teeth are inclined to the direction ofrotation to convey the waste inwardly or outwardly and connect thelatter where the openings 21 are provided inwardly of the outer reach ofthe arms; the outermost teeth 17 may be designed to induce the waste toflow inwardly while the remaining teeth promote an outward movement ofthe waste.

The hollow shaft 15 is received in bearings 29 and 30 and is driven by amotor 23 via a speed-reducing transmission 22 and a gear chainrepresented by a gear 25 on the shaft and a pinion 24 driving same whilebeing connected to the transmission 22. The hollow shaft 15 and the arms16 are cooled by air delivered by conduit 27 and outdrawn through anoutlet 26a. The inlet 26 is a hood comminuting with the spaced-apartopening 15a in the mantle 15b of the hollow shaft which has an innertube 15b separating the air-discharge path from the air-inlet path. Ateach of the arms, a pair of supply and discharge passages for the air isalso provided. The heated air used to cool the hollow shaft is collectedat conduit 28 and is delivered, together with a bypassed portion ofheated air from the blower 6, to an injector or jet fan or blower 31which communicates via line 31a with an annular duct 33 communicatingwith tuyeres 33a trained at the fluidized bed. The device 31 thusinduces gases generated in the multiple-hearth dryer in a recycling pathinto the fluidized bed. The bypassed portion of the air from blower 6 iscontrolled by a pair of valves 6b and 60. Behind these valves, theblower 6 is connected by a line 6d with a heat exchanger 10, the outflowpipe 6e of which communicates with line 28. The heat exchanger 10 istraversed by the exhaust gas.

The waste gas from the drying process emerges at the top 32 of thefurnace and includes gases evolved from the dried and preheated wastesand is entrained by the gas of line 28 to the furnace chamber. The jetfan or injector 31, operating in accordance with Venturi principles,maintains the desired subatmospheric pressure above the hearth. Theexhaust-gas temperature of at least 800C is maintained by thetemperature regulator 7b already noted, or by an oxygen detector 35which controls at 34 the oxygen introduced into the combustion chamberwhen the exhaust gas shows insufficient or excessive air. In general, Iprefer to maintain some excess of air in the exhaust gas.

The combustion gas which is not used for drying, is removed from thefluidized-bed furnace through the flue 9, is caught in the heatexchanger 10 and is then discharged through a suction fan and a flue-gascleaner.

SPECIFIC EXAMPLE The combustion of clarifier sludge is carried out in anapparatus as illustrated in FIGS. 1 and 2 in which the fluidized bedchamber has a grate diameter of 1.8 meters and a grate surface area of2.6 meters. One meter above the grate, the fluidized bed chamber widensto a diameter of 2.2m. The overall height of the fluidized bed chamberis approximately 8m, measured from the top of the grate. to the upperend of the combustion chamber. The drying zone provided above thefluidized bed chamber comprises four stages as illustrated, each with aninternal diameter of 2.2m and an effective drying surface area of 3.0m.The spacing of the individual stages from one another (vertically) isabout 0.5m.

Through the feed device 14a, 2.25 tons per hour of clarifier sludge froma water-treatment installation of a municipality, is introduced with asolids content of 25 percent. The sludge has a heat value of about 875kcal/kg. The sludge is dried on the individual hearths 13a, 13b, 13c and13d and eventually cascades through openings 21 into the fluidized bedchamber there below. The drying gas passing upwardly through theopenings 21 has a temperature of 850C and a volume of 2,600 m /hour(standard temperature and pressure STP). About 40 percent by volume ofthe gases formed in the fluidized bed chamber are supplied in thismanner to the drying zone. During passage through the drying zone, thisexhaust gas is cooled to about 350C and evaporates about 0.72 tons ofwater per hour. The solids content increases as a result of theevaporation of water at 36.7 percent.

Upon entry of the predried sludge into the fluidized bed chamber, auniform distribution of the solids is observed over the entire diameterof the chamber and the grate diameter. This distribution appears in partto be a result of the relatively long free-fall distance of the solidsfrom the openings 21. In this free-fall period, the residual water isevaporated at a rate of 0.777 tons per hour and combustion occurs withat least part of the clarifier sludge in the fluidized bed chamber.

To maintain the fluidized bed, quartz sand serves as a contact mediumand heat storage and fluidizing material and 2,600 m lhour STP of air isintroduced as the fluidizing gas. This corresponds to 1,000 m /hour(STP) per m of grate area. The air excess over that stoichiometricallyrequired to burn all of the solids of the clarifier sludge is about 1.5expressed in terms of the ratio between the available oxygen and thestoichiometrically required oxygen. An additional 550 m /hour of freshair is introduced into the fluidized bed. The fresh air is supplied bythe blower 6 through control valve 6b and then through the hollow shaft15. Aside from the fresh air, I also recycle part of the exhaust gaswithdrawn from the drying zone as a secondary source of air passingthrough a hot-gas blower 31 or a radiator.

5,550 m /hour of exhaust gas at a temperature of 850C is withdrawn fromthe fluidized bed chamber through the opening 9. Upon passage throughthe heat exchanger 10, the waste gas temperature is cooled to about 640Cand thus heats the fluidizing air from C to 575C. The heated fluidizedair is returned to the grate 2 via the burning chamber 4. Fuel oir canbe supplied to the burner chamber 4 but is not always necessary. Inpractice, combustion occurs without the supply fuel. Similarly fuel maybe supplied at pipes 7a and 1 la during the operation. In this example,the supply of further fresh air can be avoided so that pipe 34 maysimply be cut off.

I claim:

1. A method of burning a sludge containing combustible solids comprisingthe steps of:

simultaneously drying the sludge to produce solids and comminuting saidsolids by contacting the flowable material with an exhaust gas bypassing the sludge onto a plurality of vertically spaced platforms andscraping material from said platforms while inducing the material topass downwardly from one platform to another distributing the comminutedsolids over the cross-section of a fluidized bed directly below saidplatforms by causing the material to fall freely from the lowestplatform onto said bed; burning said solids in said fluidized bed in thepresence of oxygen-containing fluidized gas to produce said exhaustgases and causing said exhaust gases to flow upwardly over the materialon said platform; and recovering gases evolved from said material uponthe drying thereof and and recycling said gas to said fluidized bed.

2. The method defined in claim 1 wherein said material is a sludgeobtained from the treatment of water or sewage.

3. The method defined in claim 2 wherein said fluidized bed is a bed ofquartz and is maintained at a temperature such that said exhaust gas isat a temperature of 800 to 900C, said method further comprising the stepof releasing to the atmosphere gases contacting said material only fromabove said fluidized bed.

4. The method defined in claim 3 wherein the recycled gas is introducedinto said fluidized bed substantially in the region of the top thereof.

5. An apparatus for the combustion of a flowable material containingorganic solids, comprising housing means forming a fluidized-bedchamber, a flow chamber above said fluidized bed chamber and a dryingzone above said flow chamber; a grate below the fluidizedbed chamber;means for introducing an oxygencontaining fluidizing gas through saidgrate and into a fluidized bed sustained thereabove for burning organicsolids cascading through said flow chamber onto said fluidized bed; aplurality of vertically spaced hearths in said drying zone; means forintroducing said flowable material containing said organic solids at anupper one of said hearths; rotary take means sweeping across saidhearths for comminuting said solids while drying same by passing saidsolids downwardly from hearth to hearth in said zone; means fordischarging dried comminutd solids from the lowermost hearth onto saidfluidized bed, and for passing an exhaust gas rising from said fluidizedbed through said zone to dry the solids therein; and means forcollecting gases in said zone and recycling same to said fluidized bed.

6. The apparatus defined in claim 5 wherein the lastmentioned meansincludes a source of compressed air and suction means operated by saidcompressed air for maintaining a subatmospheric pressure above saidhearths.

7. The apparatus defined in claim 6 wherein said hearths are providedalternately with downcomers at their inner and outer peripheries, saidrake means including an air-cooled hollow shaft extending centrallythrough said zone and having radial arms sweeping over each of saidhearths, said arms being provided with rake teeth inclined to thedirection of rotation for sweeping said solids toward the respectivedowncomer, said grate being provided with a wind box connected to saidsource, said apparatus further comprising means for venting said housingmeans to the atmosphere only at said flow chamber.

* I I I

1. A method of burning a sludge containing combustible solids comprisingthe steps of: simultaneously drying the sludge to produce solids andcomminuting said solids by contacting the flowable material with anexhaust gas by passing the sludge onto a plurality of vertically spacedplatforms and scraping material from said platforms while inducing thematerial to pass downwardly from one platform to another distributingthe comminuted solids over the cross-section of a fluidized bed directlybelow said platforms by causing the material to fall freely from thelowest platform onto said bed; burning said solids in said fluidized bedin the presence of oxygen-containing fluidized gas to produce saidexhaust gases and causing said exhaust gases to flow upwardly over thematerial on said platform; and recovering gases evolved from saidmaterial upon the drying thereof and and recycling said gas to saidfluidized bed.
 2. The method defined in claim 1 wherein said material isa sludge obtained from the treatment of water or sewage.
 3. The methoddefined in claim 2 wherein said fluidized bed is a bed of quartz and ismaintained at a temperature such that said exhaust gas is at atemperature of 800* to 900*C, said method further comprising the step ofreleasing to the atmosphere gases contacting said material only fromabove said fluidized bed.
 4. The method defined in claim 3 wherein therecycled gas is introduced into said fluidized bed substantially in theregion of the top thereof.
 5. An apparatus for the combustion of aflowable material containing organic solids, comprising housing meansforming a fluidized-bed chamber, a flow chamber above said fluidized bedchamber and a drying zone above said flow chamber; a grate below thefluidized-bed chamber; means for introducing an oxygen-containingfluidizing gas through said grate and into a fluidized bed sustainedthereabove for burning organic solids cascading through said flowchamber onto said fluidized bed; a plurality of vertically spacedhearths in said drying zone; means for introducing said flowablematerial containing said organic solids at an upper one of said hearths;rotary take means sweeping across said hearths for comminuting saidsolids while drying same by passing said solids downwardly from hearthto hearth in said zone; means for discharging dried comminutd solidsfrom the lowermost hearth onto said fluidized bed, and for passing anexhaust gas rising from said fluidized bed through said zone to dry thesolids therein; and means for collecting gases in said zone andrecycling same to said fluidized bed.
 6. The apparatus defined in claim5 wherein the last-mentioned means includes a source of compressed airand suction means operated by said compressed air for maintaining asubatmospheric pressure above said hearths.
 7. The apparatus defined inclaim 6 wherein said hearths are provided alternately with downcomers attheir inner and outer peripheries, said rake means including anair-cooled hollow shaft extending centrally through said zone and havingradial arms sweeping over each of said hearths, said arms being providedwith rake teeth inclined to the direction of rotation for sweeping saidsolids toward the respective downcomer, said grate being provided with awind box connected to said source, said apparatuS further comprisingmeans for venting said housing means to the atmosphere only at said flowchamber.